This disclosure relates to a method for the manufacture of a polyalkylene terephthalate such as polybutylene terephthalate from recycled polyethylene terephthalate. In particular, the method comprises employing a titanium-containing catalyst formed by the reaction product of tetraalkyl titanate and a complexing agent comprising a phosphorous, nitrogen or boron atom.
Polybutylene terephthalate (PBT) is a well-known resin that is semi-crystalline and has several desirable properties. Compared to amorphous resins such as acrylonitrile butadiene styrene (ABS), polycarbonate, and polystyrene, a crystalline resin like PBT will show greater solvent resistance, strength, and stiffness, due to the presence of crystalline spherulites in the resin. PBT resin is used in many applications where its solvent resistance, strength, rigidity, and rigidity are needed, commonly in durable goods that are formed by injection molding.
Applications include electronic and communications equipment, computers, televisions, kitchen and household appliances, industrial equipment, lighting systems, gardening and agricultural equipment, pumps, medical devices, food handling systems, handles, power and hand tools, bobbins and spindles, and automotive parts in both under-the-hood and exterior applications. Additionally, PBT is very widely used to form electrical connectors. Through its many blended products, PBT can be tailored to a wide variety of applications.
Although useful to many customers, conventional PBT molding compositions generally cannot be made from recycled sources of PBT due to the lack of availability of large post-consumer or post-industrial supplies of PBT scrap materials. Polyethylene terephthalate (PET), unlike PBT, is made in much larger quantities and is more easily recovered from consumer wastes or the like.
With increasing demand for conserving non-renewable resources and more effectively recycling underutilized scrap PET, improved and less costly processes for deriving PBT or other polyalkylene terephthalates from scrap PET materials have been sought, in particular if the resulting derived polyalkylene terephthalate compositions possess desirable physical properties such as tensile strength, impact strength, and thermal properties.
Polyalkylene terephthalate made from recycled or scrap PET is herein referred to as “modified polyalkylene terephthalate,” including “modified PBT,” wherein the polymer is modified by containing at least one residue derived from the polyethylene terephthalate component used in the process. The residue can be either incorporated into the structure of the polymer or present in admixture with the resin composition. Thus, the modified polyalkylene terephthalates can identifiably differ slightly from virgin PBT by such modifications which, however, can be controlled so that the modified PBT has desirable properties comparable or similar to virgin PBT with little or no adverse effects.
The modified polyalkylene terephthalate can generally be made by reacting alkylene diol such as 1,4-butanediol with polyethylene terephthalate (PET) particulates, for example flakes, in the presence of a transesterification catalyst. U.S. Pat. Nos. 7,129,301; 6,020,393; 4,328,059, and US Patent Pub. 2005/0113534 A1 disclose various catalysts for the polymerization of polyesters. Tetraalkyl titanates have been commonly used as catalysts for PBT polymerization. The various titanates can include tetraisopropyl titanate, tetrabutyl titanate, and tetra(2-ethylhexyl) titanate. JP 60147430 discloses a method of producing polyester by esterifying terephthalic acid, adipic acid and 1,4-butanediol in the presence of titanium compound and a pentavalent phosphorus compound. U.S. Pat. No. 6,303,738 B1 discloses a process for producing copolyester containing adipic acid in the presence of TYZOR IAM (available from DuPont), which was prepared through the combination of TPT (tetraisopropyl titanate) and a mixture of butyl phosphate and dibutyl phosphate. These catalysts, however, have not been used for the production of modified polyalkylene terephthalates from PET.
At the end of the polymerization process, the catalyst employed for the polymerization of the polyalkylene terephthalate is typically not quenched (deactivated) in the resulting resin composition. Unfortunately, an active catalyst in the resin composition can sometimes lead to undesirable reactions in subsequent processing of the modified polyalkylene terephthalate. Blends containing the modified polyalkylene terephthalate, on exposure to high temperature and humidity, can exhibit hydrolytic degradation, especially under caustic conditions. Another problem associated with some blends is transesterification, which can lead to loss of mechanical properties. Catalyst quenchers can be added to thermoplastic compositions to prevent such transesterification, but such catalyst quenchers can also promote degradation of polymer chains and contribute to a decrease in hydrolytic stability. Conventional phosphorous derivatives such as phosphoric acid, phosphates have been used as quenchers. The use of phosphite stabilizers is less satisfactory because of the tendency to be unstable to both hydrolysis and oxidation.
Insufficient hydrostability can lead to chain cleavage, to which extent depending on the exact conditions of exposure to water or humidity. Temperature, time of exposure, and pH are all factors. Both acids and bases can catalyze ester hydrolysis. If the water is acidic or basic, or the polymer matrix involves free acidic or basic additives, decomposition can be accelerated. Since one of the reaction products of polyalkylene terephthalates hydrolysis is itself a carboxylic acid, the hydrolytic decomposition of a polyalkylene terephthalates such as PBT is autocatalytic.
Thus, there remains a need for new and improved catalysts or synthetic approaches for the production of polyalkylene terephthalates that are effective in polymerization, but which do not have adverse effects in the resin composition, during later processing or in polymer blends, after polymerization has been completed.